The invention relates to a coating composition suitable in particular for producing coatings on automobile parts. The parts thus coated, especially those where the coating composition described here is used to produce a surfacer coat, are notable for especially high stone-chip resistance.
In DE-A 39 18 510, conventional baking surfacers based on blocked isocyanates and polyesters (in solution in organic solvents) are described.
In EP-A 0 249 727, aqueous surfacer coating compositions are described based on phosphoric-acid-modified epoxy resins, water-dilutable polyesters as known from AT-B 328 587, and water-dilutable melamine resins. DE-A 40 00 748 relates to aqueous surfacer coating compositions comprising water-dilutable hydroxyl-containing polyurethane resins, which may if desired include other, admixed binders, and aqueous amino resins as curing agents. Mixtures of water-dilutable polyurethane resins, of water-dilutable polyesters modified with epoxy resin, and, if desired, of water-dilutable amino resins are known from DE-A 38 13 866. Other aqueous surfacer coating compositions based on carboxyl-containing polyesters or acrylic copolymers and water-soluble blocked isocyanate prepolymers, alone or in combination with water-dilutable amine resins, are described in DE-A 38 05 629. EP 0 594 685 relates to the use of condensation products of carboxyl-containing polyurethane resins and hydroxyl-containing polyester resins, with or without urethane modification, together with water-insoluble blocked isocyanates for producing stoving enamels. An improvement of this formulation with reduced sensitivity to so-called overbaking is known from EP-A 0 548 873, the improvement being achieved by adding a water-soluble amine resin as crosslinker.
Further-improved systems for aqueous surfacers are described, for example, in applications AT 2171/99, AT 2172/99, and AT 2173/99. Although these binders are improved in particular with respect to solids content and also in the stone-chip resistance, weaknesses are still evident. There is therefore evidently still a need for further optimization of the cured paint film in respect of its elasticity and strength.
It is the object of the invention, therefore, to make further improvements in the stone-chip resistance of aqueous binders for surfacer materials which are used in the line production of automobiles.
It has now been found that adding an aqueous dispersion of a particularly high molecular mass polyurethane to condensation products of hydroxyl-containing and carboxyl-containing resins and combining this mixture with curing agents which become active on heating make it possible to obtain aqueous coating compositions whose stone-chip resistance after baking is improved relative to that of the known systems.
The invention accordingly provides a coating composition comprising
a condensation product A of a carboxyl-containing resin A1 and a hydroxyl-containing resin A2, A1 preferably having an acid number of from 100 to 230 mg/g, in particular from 120 to 160 mg/g, and A2 preferably having a hydroxyl number of from 50 to 500 mg/g, in particular from 60 to 350 mg/g,
a water-soluble or water-dispersible, high molecular mass polyurethane B having a number-average molar mass Mn (measured by gel permeation chromatography, calibration with polystyrene standards) of at least 10 kg/mol, preferably at least 15, and with particular preference at least 20 kg/mol, and a weight-average molar mass Mw of at least 20 kg/mol, preferably at least 30, and with particular preference at least 40 kg/mol, the acid number of the polyurethanes B being preferably from 20 to 50 mg/g, in particular from 25 to 45 mg/g, and
a curing agent C which becomes active only at an elevated temperature of at least 80xc2x0 C.